There are several ways to increase availability with parallel redundant servers that provide both load balancing and failover. The following figure illustrates two replicate servers providing an N+1 failover system. An N+1 system has an additional server to provide 100% capacity should one server fail.
The computing power of each server in Horizontally Redundant Systems above is identical. One server alone handles the performance requirements. The other server provides 100% of the performance when called into service as a backup.
The advantage of an N+1 failover design is 100% performance during a failover situation. Disadvantages include increased hardware costs with no corresponding gain in overall performance (because one server is a standby for use in failover situations only).
In the system depicted in Horizontally Redundant Systems above, if one server fails, all services are available, although at a percentage of the full capacity. The remaining server provides 6 CPUs of computing power, which is 60% of the 10 CPU requirement.
An advantage of this design is the additional 2 CPU latent capacity when both servers are available.
The following figure illustrates a distribution between a number of servers for performance and load balancing.
Because there are five servers in the design depicted in Horizontally Redundant Systems, if one server fails the remaining servers provide a total of 8 CPUs of computing power, which is 80% of the 10 CPU performance requirement. If you add an additional server with a 2-CPU capacity to the design, you effectively have an N+1 design. If one server fails, 100% of the performance requirement is met by the remaining servers.
This design includes the following advantages:
Added performance if a single server fails
Availability even when more than one server is down
Servers can be rotated out of service for maintenance and upgrades
Multiple low-end servers typically cost less than a single high-end server
However, administration and maintenance costs can increase significantly with additional servers. You also have to consider costs for hosting the servers in a data center. At some point you run into diminishing returns by adding additional servers.